Minimum requirements for a custom libc

[Johannes Krottmayer]

Dear GCC community!

I have started a custom libc implementation for the purpose of
education.

What are the minimum requirements for the library (functions,
headers, ...) which one *must* be implemented so GCC can use this 
library as default?

And how should I write the specification file, so that GCC can use this
library in the build process from GCC?

How to deal with multi architecture support in a custom libc? What
are here the requirements?

Thanks in advance!

Kind regards,

Johannes K.